This thesis aims to investigate the characteristics and structural properties of bolted connections in steel structures with the main focus on the design adequacy and failure mechanisms of high-strength friction-grip (HSFG) bolted connections. As a case study, the bolted connections on the Mary River Bridge along Arnhem Highway in the Northern Territory have been investigated. The composite highway bridge is composed of reinforced concrete deck over five simply supported spans of structural steelwork. The structural steelwork includes five main UB girders connected to diaphragms and horizontal bracings by HSFG bolted connections. Initial inspection of the bridge found that a number of bolts were loosened, missing or had already fractured and most were heavily corroded. Remedial works in which the original bolted connections have been replaced and the new ones ensured to be installed at the correct tension have been recently completed. The structural and environmental factors that may have contributed to the eventual failure of the bolts have been investigated. The design adequacy of the bolted connections compared to the externally applied loads on the bridge superstructure was checked in accordance with current standards: AS5100 and AS4100. The failure mechanisms of the fractured bolts were investigated. The residual loads on the original bolts have been identified through slip testing on both the original bolts and new HSFG bolts in double shear configuration. The fractured bolts were analysed through optical and scanned electron microscopy. The design fatigue life of the bolts were identified through fatigue testing of the M16 bolts in double shear configuration and the M22 bolts subjected to cyclic tensile load. The difference of the rate of corrosion of the bolts with and without the zinc plate corrosion protection was also identified. It was found that the main factors the contributed to the failure of the bolted connections at the Mary River Bridge include overstressing of the bolts, the fluctuating loads, the eventual abrasion and wear of the corrosive protection and the corrosive environment it was subjected to. The failure mechanisms of the fractured bolts were mainly due to corrosion and fretting fatigue. Similarly, the bolts subjected to tension, have failed due to self-loosening over time.